Hydrogels prepared from photo-polymerizations have been an attractive class of biomaterials for tissue engineering and regenerative medicine applications. Radical initiated photo-polymerizations have also received significant attention for in situ encapsulation and delivery of biologics, including cells, proteins, RNAs, and DNAs. In general, photo-polymerizations for hydrogel synthesis are initiated by either long wavelength ultraviolet light (e.g., λ=365 nm) using a type I initiator (e.g., Irgacure 2959 or lithium arylphosphanate), or alternatively using visible light (λ=400-700 nm) with a type II initiator (e.g., eosin-Y) and appropriate co-initiator/co-monomer. Following light exposure, a type I (cleavage type) photoinitiator readily absorbs photons and decomposes into two primary radicals, which participate in the polymerization process. In contrast, a type II photoinitiator absorbs photons to achieve an excited state, which abstracts a hydrogen atom from a reactant to form a secondary radical, which participates in the polymerization process. UV-mediated reaction is generally a preferred method for preparing hydrogels due to the simplicity in preparing polymer precursor solutions, as well as the rapid and spatial-temporally controlled gelation kinetics. However, visible light-mediated gelation is believed herein to be more desirable for biomedical applications due to the lower potential for light-induced damage.
For example, even at long wavelengths (e.g., λ=365 nm), the use of UV light for biomedical applications may raise biosafety concerns. Though, conventional visible light-mediated polymerization is more desirable, the utility of conventional visible light-mediated polymerization is limited by its slow reaction kinetics, and the general necessity to add co-monomers and co-initiators, each of which may be cytotoxic. For example, visible light sources (λ=400 to 700 nm) can be used together with appropriate photoinitiators (e.g., eosin-Y) to initiate photopolymerizations. When eosin-Y is excited by visible light, it abstracts hydrogen atoms from a co-initiator triethanolamine (TEOA) to form secondary radicals. These radicals then propagate through vinyl groups on the macromers (e.g., poly(ethylene glycol) diacrylate or PEGDA) to form a crosslinked polymer network. Unfortunately, the photopolymerization kinetics using eosin-Y and TEOA are slow and therefore a co-monomer, such as 1-vinyl-2 pyrrolidinone (NVP) is commonly added to accelerate the gelation kinetics. These additional components (TEOA and NVP) make adjusting the compositions of precursor solution more complicated. In addition, the use of co-monomers may be unaccompanied by undesirable cytotoxicity.
Further, two general criteria, namely solubility in water and molar absorptivity at cytocompatible wavelengths (λ=360 nm), are used to evaluate the suitability of a photoinitiator in biomedical applications. Thus, only a few photoinitiators have been reported to fulfill these criteria and are considered cytocompatible to be used in preparing hydrogels for in situ cell encapsulation. Those photoinitiators include type I initiators, such as Irgacure-2959 (I-2959) and lithium arylphosphinate (LAP), and type II initiators, such as eosin-Y (a fluorescent red dye for histological staining). Commercially available I-2959 has low water solubility (<0.5 wt %) and low molar absorptivity at 365 nm (ε<10 M−1 cm−1). Furthermore, I-2959 cannot be used in visible light-mediated photocrosslinking systems due to its near zero molar absorptivity at wavelengths higher than 400 nm. Even though LAP is highly water-soluble (>5 wt %) and has higher molar absorptivity at 365 nm (ε˜200 M−1 cm−1), its utility in visible light range is also limited (ε˜30 M−1 cm−1 at 405 nm). In contrast, type II photoinitiators, such as eosin-derivatives, are highly water-soluble and can be readily excited by visible light (λ=400 to 700 nm). Although eosin-Y itself is not cytotoxic, the use of eosin-Y is hampered by the need to include one or more toxic co-initiators and accelerants to generate sufficient radicals to achieve high functional group conversion and yield stable crosslinked hydrogels.
Accordingly, there is a need for new compositions for preparing hydrogels using visible light-mediated polymerization without the need to add cytotoxic co-monomers.
It has been discovered that the hydrogel precursor mixtures described herein undergo rapid visible light-mediated polymerization, also termed gelation herein, and are useful for preparing cytocompatible thiol-ene photo-click hydrogels. The hydrogel precursor mixtures include cytocompatible visible light photoinitiators, such as eosin-Y (EY), and do not require adding any cytotoxic components to achieve rapid gelation under ambient conditions. It has also been discovered herein that multi-layered thiol-ene hydrogels can be fabricated using a surface-mediated thiol-ene photopolymerization. It has also been discovered herein that such multi-layered thiol-ene hydrogels may be prepared by manipulating the diffusion rate of the initiator, such as eosin-Y.
In one illustrative embodiment of the invention, hydrogel precursor mixtures capable of forming a hydrogel are described herein. In one aspect, the mixtures include one or more macromers, each comprising a carbon-carbon multiple bond, one or more crosslinking agents, and a type II photoinitiator having at least one peak absorbance in the visible light region.
In another embodiment, hydrogels are described herein that are prepared from the hydrogel precursor mixtures described herein.
In another embodiment, hydrogel delivery systems are described herein. In another embodiment, hydrogel delivery systems are described herein that comprise a single hydrogel layer. In another embodiment, hydrogel delivery systems are described herein that comprise two or more hydrogel layers. In one variation, each layer of the multiple layer hydrogels have the same or a similar composition, In another variation, two or more have a different composition from each other. The hydrogel delivery systems may include one or more populations of cells, one or more therapeutic agents, one or more diagnostic agents, or any combination of the foregoing.
It is appreciated herein that the delivery systems described herein may be used alone or in combination with other compounds useful for treating or diagnosing diseases.